Fix exportdecl validation rule and sync with export

Author: lukewagner

design/mvp/Binary.md

@@ -260,8 +260,8 @@ Notes:
 * Validation rejects `resourcetype` type definitions inside `componenttype` and
   `instancettype`. Thus, handle types inside a `componenttype` can only refer
   to resource types that are imported or exported.
-* Validation requires all types used in the type of an export are introduced
-  by an `exportdecl` (not an `importdecl`).
+* Validation requires that all resource types transitively used in the type of an
+  export are introduced by a preceding `exportdecl`.
 * The uniqueness validation rules for `externname` described below are also
   applied at the instance- and component-type level.
 * Validation of `externdesc` requires the various `typeidx` type constructors
@@ -334,8 +334,8 @@ URL        ::= b*:vec(byte)                    => char(b)*, if char(b)* parses a
 Notes:
 * All exports (of all `sort`s) introduce a new index that aliases the exported
   definition and can be used by all subsequent definitions just like an alias.
-* Validation requires that any handle type that is part of the type of any
-  export must refer to a resource type whose index was produced by an `export`.
+* Validation requires that all resource types transitively used in the type of an
+  export are introduced by a preceding `exportdecl`.
 * The "parses as a URL" condition is defined by executing the [basic URL
   parser] with `char(b)*` as *input*, no optional parameters and non-fatal
   validation errors (which coincides with definition of `URL` in JS and `rust-url`).